1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, and more specifically to an air intake side secondary air supply system in which the operation of the system is improved when the amount of the intake air is small.
2. Description of Background Information
Air/fuel ratio feedback control systems for an internal combustion engine are known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and an air/fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy. As an example of the air/fuel ratio feedback control system, an air-intake side secondary air supply system of a duty ratio control type is proposed, for example, in Japanese Patent Publication No. 55-3533 in which an open-close valve is disposed in an air intake side secondary air supply passage leading to a part of an intake manifold, downstream of a throttle valve of a carburetor, and a duty ratio of the open and close of the open-close valve, i.e. the supply of the air intake side secondary air, is feedback controlled in response to the output signal level of the O.sub.2 sensor.
In such an air-intake side secondary air supply system of the duty ratio control type, the amount of the secondary air flowing through the open-close valve changes very little with respect to the change in the control signal in a small range (0.about.20%, for example) of the duty ratio which indicates a time proportion of the opening of the open-close valve in each of the duty period. However, under a condition in which the engine speed is low and the vacuum level in an intake manifold is high, e.g. when the engine is idling, the amount of air flowing through the throttle valve is relatively small, and consequently, the amount of the secondary air required for controlling the air/fuel ratio becomes also small. For this reason, the duty ratio must often be controlled into the small range. Thus, when the amount of the engine intake air is small, the air/fuel ratio control may become inaccurate with the conventional air intake side secondary air supply systems.
On the other hand, there is an air intake side secondary system in which a linear type solenoid valve is provided in the air intake side secondary air supply passage leading to the intake manifold. Such an air intake side secondary air supply system is disclosed, for example, in Japanese Patent Application laid-open No. 55-119941. In this system, an opening degree of the linear type solenoid valve is varies in response to the magnitude of a drive current supplied to its solenoid. With this solenoid valve, a sectional area of the air intake side secondary air supply passage is varied in response to a result of detection of the oxygen concentration in the exhaust gas.
In this type of air intake side secondary air supply system, the amount of the intake air as well as the amount of the secondary air for controlling the air/fuel ratio becomes small when the enginee speed is low and the vacuum in the intake manifold is high, for example, when the engine is idling. Therefore, if the linear type solenoid valve is constructed such that the openning degree increases as an increase of the drive current, the magnitude of the drive current to the solenoid valve becomes small under a condition mentioned above. However, in general, in the case of the linear type solenoid valve, the opening degree does not necessarily vary accurately in proportion to the change in the drive current. More specifically, the change in the opening degree per a unit current value becomes small when the magnitude of the drive current is small. Therefore, with conventional air/fuel ratio control systems of this type, the amount of the secondary air may deviate from the proper value, to reduce the accuracy of the air/fuel ratio control when the amount of the intake air of the engne is small. This is especially serious if the amount of the drive current is determined digitally by using a microcomputer having a CPU, and the resolution of the control is not high enough, or in other words, only a coarse control is performed by the digital control.